Smartglasses are a special form of a Head Mounted Display. One conventional form of Head Mounted Displays uses screens that are worn in front of the eyes and present the user with computer-generated images or images recorded by cameras. Such Head Mounted Displays are often voluminous and do not allow direct perception of the surroundings. It is only relatively recently that Head Mounted Displays have been developed which are able to present the user with an image recorded by a camera or a computer-generated image without preventing direct perception of the surroundings. Such Head Mounted Displays, which are referred to as smartglasses hereinafter enable this technology to be utilized in everyday life.
Smartglasses can be provided in various types. One type of smartglasses, which is distinguished in particular by its compactness and aesthetic acceptance, is based on the principle of waveguiding in the spectacle lens. In this case, light generated by an image generator is collimated outside the spectacle lens and coupled in via the end face of the spectacle lens, from where it propagates via multiple total internal reflection to a point in front of the eye. An optical element situated there then couples out the light in the direction of the eye pupil. In this case, the input coupling into the spectacle lens and the output coupling from the spectacle lens can take place either diffractively, reflectively or refractively. In the case of diffractive input or output coupling, diffraction gratings having approximately the same number of lines are used as input and output coupling elements, the greatly dispersive effects of the individual gratings being compensated for among one another. Input and output coupling elements based on diffraction gratings are described for example in US 2006/0126181 A1 and in US 2010/0220295 A1. Examples of smartglasses comprising reflective or refractive input or output coupling elements are described in US 2012/0002294 A1.
Smartglasses in which an imaging beam is guided with multiple reflection from an input coupling element to an output coupling element, irrespective of whether diffractive, reflective or refractive elements are used as input and output coupling elements, have in common the problem of the so-called “Footprint Overlap”. This problem, which limits the size of the field of view (FOV) and the size of the exit pupil of the smartglasses at the location of the eyebox and on account of which a relatively high spectacle lens thickness is necessary, is explained in greater detail below with reference to FIGS. 5 and 6.
The eyebox is that three-dimensional region of the light tube in the imaging beam path in which the eye pupil can move, without vignetting of the image taking place. Since, in the case of smartglasses, the distance of the eye with respect to the smartglasses is substantially constant, the eyebox can be reduced to a two-dimensional eyebox that only takes account of the rotational movements of the eye. In this case, the eyebox substantially corresponds to the exit pupil of the smartglasses at the location of the entrance pupil of the eye. The latter is generally given by the eye pupil. Although smartglasses are a system with which an imaging beam path runs from the image generator to the exit pupil of the smartglasses, for an understanding of the “Footprint Overlap” it is helpful to consider the beam path in the opposite direction, that is to say from the exit pupil to the image generator. Therefore, a light tube emanating from the exit pupil of the smartglasses is considered in the following explanations, wherein the boundaries of the light tube are determined by the field of view angles of the beams propagating from every point of the eyebox in the direction of the spectacle lens.
After refraction at the inner surface 103 of the spectacle lens 101, the rays in the light tube impinge on the outer surface 105 of the spectacle lens 101. The output coupling structure 107 is situated in said outer surface and extends in a horizontal direction from the point B to the point C. The distance between the points B and C is determined by the desired extent of the light tube, which in turn depends on the desired size of the eyebox 109 and the desired field of view angle. The field of view angle here is primarily the horizontal field of view angle, which concerns that angle relative to the axis of vision at which the horizontal marginal points of the image field are incident in the pupil. The axis of vision here denotes a straight line between the fovea of the eye (point of sharpest vision on the retina) and the midpoint of the image field. FIG. 5 illustrates the profile of the light tube given an eyebox diameter E and a thickness d of the spectacle lens 101 for a relatively small field of view angle. All rays of the light tube are diffracted or reflected from the output coupling structure 107 in the direction of the inner surface 103 of the spectacle lens 101 and from there are reflected back to the outer surface 105 of the spectacle lens 101, from where they are reflected back again onto the inner surface 103 of the spectacle lens 101. This reflection back and forth takes place until the input coupling element is reached, from where the light tube then progresses further in the direction of the image generator.
If, as illustrated in FIG. 5, the field of view angle is relatively small, the rays of the light tube, after the first reflection at the inner surface 103 of the spectacle lens 101, impinge on a region of the outer surface 105 of the spectacle lens 1 which lies outside the output coupling element 107 (in FIG. 5 on the right next to the point B). By contrast, if a large field of view angle is desired, as is illustrated in FIG. 6, a correspondingly enlarged output coupling structure 107′ is necessary. However, this has the effect that rays of the light tube which impinge on that section of the output coupling structure 107′ which is located between the points A and C, after the first reflection at the inner surface 103 of the spectacle lens 101, are reflected back onto a region of the outer surface 105 of the spectacle lens 101 in which the output coupling structure 107′ is still situated. This region, referred to hereinafter as overlap region, is situated between the points B and D in FIG. 6. Owing to the presence of the output coupling element, which may be a diffractive or reflective output coupling element in the illustration selected in FIG. 6, the rays reflected from the inner surface 103 of the spectacle lens 101 into the region between B and D are not reflected back in the direction of the inner surface 103, such that they are lost for the imaging.
A similar problem also occurs if the diameter of the eyebox is increased rather than the field of view angle. In this case, too, there would be points A and C between which there is situated a region which reflects rays in the direction of the inner surface 103 of the spectacle lens 101 which are reflected back from there once again into a region of the output coupling structure 107′ that is identified by the points B and D, and are therefore unusable for the imaging. The same would also correspondingly hold true if the eyebox diameter E and the field of view angle were maintained and in return the thickness d of the spectacle lens were reduced. In other words, a sufficiently large eyebox diameter E in conjunction with a sufficiently large field of view angle can be achieved only with a certain minimum thickness d of the spectacle lens.
It should be pointed out once again at this juncture that the beam path was reversed for the above consideration, and that the actual beam path runs from the image generator into the exit pupil of the smartglasses. This does not change anything about the fundamental consideration, however, since rays which come from the image generator and which impinge on the output coupling structure 107′ in the region between the points B and D are not reflected into the exit pupil since they are not reflected back in the direction of the inner surface of the spectacle lens, which would be necessary, however, in order to reach the region of the output coupling structure 107′ between the points A and C, from where they could be coupled out in the direction of the exit pupil.